The present invention relates to a community antenna television (CATV) communication system in which a distribution unit is interposed between a center and each subscriber's terminal so that channel selecting control for the terminal is performed in the distribution unit.
FIG. 1 shows a conventional communication system, in which a TV signal transmitted to a transmission system 4 from a head end 1 at a center side through a trunk amplifier 2 and a distribution amplifier 3 is distributed to respective transmission lines 115 for corresponding terminals 500 through distributors 117 and 118. In each terminal 500, a data signal superimposed on the TV signal is separated from the latter through a filter 501, and the TV signal is transferred to a descrambler 505 through a converter 504 so as to be descrambled thereat, and is then transferred to a TV receiver 600. On the other hand, the data signal is demodulated by a demodulator 502 and supplied to a controller 506. Upon reception of a channel selection signal selected by the key operation of a remote controller 508, the controller 506 judges whether the channel is permitted to be received or not, drives a PLL circuit 503 to perform electronic control of tuning of the converter 504, and causes the descrambler 505 to operate.
The terminal 500 has, as described above, the converter 504, the controller 506 for controlling the converter 504 through the PLL circuit 503, and the descrambler 505, thereby performing data communication function on the center side, PLL control, remote input control, selected channel indication, descrambler control etc.
In this system, a problem arises in that many functions and high responses in order to respond to the polling from the center side are required for a controller in every terminal. Therefore, the cost of each terminal is considerably expensive. Further there is a disadvantage that the terminals must be given individual addresses because polling is performed between the center and each terminal, and thus the time taken for polling becomes longer as the number of the terminals increases. Moreover, there is a further problem in that wire tapping reception may be easily performed by copying the design of any terminal.
Therefore, there has been proposed a distribution unit system in which channel-selection control for the terminals is concentratedly performed at a distribution portion.